In the control of a variable compression ratio mechanism, a compression ratio sensor that detects a compression ratio from a rotation angle of a control shaft of a multi-link mechanism is used as described in Japanese Patent Application Laid-open Publication No. 2006-226133 (Patent Document 1). The compression ratio sensor includes a relative angle sensor that detects a relative angle of an actuator, and an absolute angle sensor that detects an absolute angle of the control shaft coupled to an output shaft of the actuator via a speed reducer. An output value of the absolute angle sensor is obtained as a reference point at the start-up of an internal combustion engine, and thereafter, a rotation angle of the control shaft is calculated from an output value of the relative angle sensor.